Two significant advances in flow meter capabilities over the last two decades have been the reduction in flow measurement uncertainty and advances in flow meter diagnostics. These two issues are directly related. A meter's flow rate prediction uncertainty rating is only truly valid if there is a guarantee that the meter is fully serviceable. This guarantee can be supplied from the meter's diagnostic tools (or diagnostic ‘suite’).
It could be reasonably argued that over the last twenty years the advance in flow meter diagnostic suites has been more industrially significant than the reduction of flow rate prediction uncertainty. However, no flow meter has a diagnostic suite capable of identifying all (or even most) problems before those problems cause a flow rate prediction bias in excess of the stated uncertainty rating. Industry will therefore benefit from further advances in flow meter diagnostic capabilities.
The majority of single phase flow meter designs on the market today use some single specific fundamental flow metering technique. For example, Differential Pressure (DP) meters cross reference the physical laws of the conservation of mass and energy to derive the fluid flow rate. The vortex meter reads the frequency of shedding vortices off a bluff body. It relates this shedding frequency to the average fluid velocity and therefore flow rate. The ultrasonic meter takes discrete average velocity readings based on the difference in time of flight of ultrasonic waves upstream and downstream along paths in the meter body. With these readings it determines the overall average fluid velocity and hence the fluid flow rate. Much of the present marketing of each meter design consists of promoting the pros and downplaying the cons of that particular design while doing the opposite for the alternative technologies.
Each flow meter design persists on the market as it has benefits for particular applications. In many flow meter applications different technologies will each successfully and satisfactorily meter the fluid flow. Often there is little performance difference between competing flow meter technologies.
One flow meter design principle is to dispense with the limiting concept that one physical metering principle is better than another. It is known that there are merits in different physical principles. Therefore, instead of choosing one over the other, there is a principle of combining two or more physical principles. Such a hybrid meter should have the combined pros of both meters while potentially negating some of the cons of each meter.
Examples of this metering concept are the ideas of Boden (U.S. Pat. No. 2,772,567), Pfrehm (U.S. Pat. No. 3,430,489), Lisi (U.S. Pat. No. 3,785,204) and Mottram (GB 2,161,941). Boden combined a turbine meter with a Venturi DP meter. Pfrehm improved Boden's design. Lisi used Boden's principle to combine a vortex meter with different DP meters. Mottram improved on Lisi's design. These hybrid designs:                produced metering systems with redundancy—i.e. two meters in one,        produced ‘over determination’ of the flow rate—i.e. two flow rate predictions to check against each other when the two sub-systems operated independently,        allowed the outputs of different flow meters, using different physical principles, to be cross referenced thereby producing extra flow information not attainable from either stand alone meter.        
Therefore, it has been known for many years that combining different flow meter principles can produce distinct advantages for the metering of fluid flows.
However, even these hybrid meters still do not provide a diagnostic capability that can identifying all (or even most) problems before those problems cause a flow rate prediction bias in excess of the stated uncertainty rating. Industry will therefore benefit from further advances in flow meter diagnostic capabilities.